1. Field of the Invention
The present invention relates generally to photolithography methods for fabricating semiconductor wafers and other microsystems devices, and more particularly, to an improved method for controlling pattern edge profiles, and forming undercut openings, in photoresist layers applied to transparent wafer substrates.
2. Description of the Relevant Art
The art of photolithography in the fine patterning of semiconductor and other substrates is well known. Examples of applications for photolithography are the formation of integrated circuits, magnetic recording heads or other Micro Electro-Mechanical Systems (MEMS) devices. In conventional pattern forming processes, a substrate is coated with a layer of material that is sensitive to some form of electromagnetic radiation, including optical radiation, X-ray, ions and/or electron radiation. This material is usually called photoresist or polymide. Those portions of the resist layer that are exposed to such radiation undergo a change relative to solubility by a specific solvent, or developer. Accordingly, desired patterns can be formed within the resist layer as a result of the differential solubility-between the irradiated and non-irradiated regions of the photoresist layer.
The above-described solubility changes are produced by either bond-breaking (chain scission) or bond formation (chain cross-linking) in a polymeric resist. A positive-acting resist will become more soluble, via chain scission, when irradiated; accordingly, when using a positive resist, the resist material remaining after exposure and development of the resist corresponds to the opaque (non-transmissive) portion of the mask used to selectively pattern the resist. In contrast, a negative resist will cross-link, and become more insoluble, when irradiated; accordingly, when using a negative resist, the resist material remaining after exposure and development of the resist corresponds to the transparent (transmissive) portion of the mask used to selectively pattern the resist.
Of course, the whole point of creating a patterned resist layer is to be able to accurately transfer the pattern defined in the resist layer to another layer of material in or on the substrate. For example, it may be desired to chemically etch openings within an insulative layer of silicon dioxide formed upon a silicon wafer, and in this instance, the polymeric resist can shield portions of the silicon dioxide layer from the chemical etchant. As a second example, it may be desired to pattern a metal interconnect layer formed upon a semiconductor wafer, and in this case, the polymeric resist can protect covered portions of the metallic layer from being stripped away.
There are many different kinds of pattern transfer processes which can be additive or subtractive in nature. Subtractive techniques involve an etching away of material using, for example, a dry plasma, a chemical solution, or ion beam exposure. Additive processes are those where material is deposited, for example, via evaporation, electroplating or ion implantation, after the resist has been patterned. In an additive, or so-called xe2x80x9clift-off processxe2x80x9d, an added layer is deposited through the resist openings onto the substrate after resist patterning; then the resist is stripped off, leaving only the deposited layer in the regions where openings had been formed in the resist layer.
When the aforementioned additive lift-off process is used to deposit a metal layer onto a substrate, an undercut resist profile is desired to effect a clean discontinuity of the deposited metal layer. Such an undercut resist profile can be achieved by using a positive resist, and by patterning such positive resist layer using an electron beam. For example, in U.S. Pat. No. 5,468,595 issued to Livesay, two electron beam exposure methods are described wherein the electron beam is used to vary the solubility properties of the resist material depending upon the depth of the resist material, i.e., depending upon how close the region of the resist material lies to the upper surface of the substrate. By exposing the lower region of the positive resist layer to a greater dosage of the electron beam, as compared with a lesser dosage of the electron beam used to expose the upper region of the positive resist layer, the lower region of resist can be made more soluble, resulting in the formation of undercut openings in the resist layer (see Livesay ""595, column 6, lines 22-51). In the second method, it has been found that solubility reaches a maximum at a particular dosage, and that further increases in electron beam dosage actually cause solubility to decrease. In these cases, the upper region can be made less soluble than the lower region by using a higher electron beam dosage for the upper region (see Livesay ""595, column 8, lines 35-68 and column 9, lines 1-17).
One significant drawback of the process described in the ""595 Livesay patent is the creation of undesired xe2x80x9cfeetxe2x80x9d within the base portion of the patterned resist openings. The reasons for the creation of such xe2x80x9cfeetxe2x80x9d are demonstrated in Prior Art FIGS. 1-4. As shown in Prior Art FIG. 1, a substrate 20 is coated with a positive resist layer 22 and exposed by a patterned Ultraviolet light 24 (using a wafer stepper, mask aligner, or holographic exposure machine) to a desired exposure level. The exposed resist layer is then developed using a positive resist developer, resulting in the structure shown in FIG. 2, including resist opening 26. In an ideal process, the resist profile of opening 26 would exhibit sidewalls 28 that remain perfectly vertical down to substrate 20. In practice, however, when resist layer 22 is being developed, sidewalls 28 actually extend at an angle in the 85 to 90 degree range relative to the surface of substrate 20, and exhibit some radius at their feet 30.
The structure shown in Prior Art FIG. 2 is thereafter exposed to a lower-level dosage electron beam (not shown) to adjust the relative solubilities of the upper and lower regions of the resist layer. Then, as shown in Prior Art FIG. 3, and in anticipation of producing an undercut resist opening, a high dosage, low accelerating voltage, electron beam 32 is applied to the structure to render the top region 34 of resist layer 22 unsoluble. However, feet 30 are also exposed to such electron beam, and therefore become relatively unsoluble, as well; this remains true even if the structure is thereafter subjected to exposure by a lower dose, higher accelerating voltage, electron beam used to render the rest of the resist layer soluble. This problem is mentioned in the Livesay ""595 patent at column 9, lines 54-64. Following the second resist development step, the unsoluble radius feet 30 remain, as shown in FIG. 4. These feet present an unacceptable fault for purposes of using such undercut resist openings to form a patterned layer upon substrate 20 using the above-described lift-off process.
Accordingly, it is an object of the present invention to provide a method of creating undercut openings in a positive resist layer upon a substrate to facilitate patterned deposition of metal layers upon such substrate using a metal lift-off process while suppressing the formation of feet within such undercut openings.
Another object of the present invention is to decrease the sensitivity of such undercut resist opening fabrication technique to the non-uniformity of large-area electron beam equipment used to expose such resist layers.
Still another object of the present invention is to simplify the aforementioned multiple low-dose exposure step, used to selectively vary the solubility of different resist regions, especially when relatively thick photoresist is used.
A further object of the present invention is to reduce unwanted cross-linking on sidewall surfaces of the resist layer, and to improve overhang control by maintaining the lateral dissolution rate of the resist more constant.
Yet another object of the present invention is to provide such a method which does not require the use of multi-acceleration, low dosage electron beam exposure in order to vary the solubility of resist layers at different depths.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.
Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to a method of creating an undercut resist sidewall profile within an opening formed in a positive resist layer disposed upon a substrate that is relatively transparent. The method includes the steps of forming a positive resist layer upon the upper surface of the transparent substrate, and optically patterning the resist layer, as by selectively directing light through a mask at the resist layer in an area where an opening is to be formed. The lower surface of the transparent substrate is flooded with light to partially expose the lowermost region of the resist layer lying closest to the substrate; this flooding step can be performed either before or after the optical patterning step. Thereafter, the resist layer is developed a first time by dissolving exposed resist to form a patterned opening therein. Such opening has an increased width within the lowermost region of the resist layer adjacent the substrate because of the above-mentioned exposure of the lowermost region of the resist layer via the lower surface of the transparent substrate.
In the preferred embodiment of the present invention, the resulting structure is then flooded from above with light, a low dosage electron beam, or other electromagnetic radiation to expose all the remaining photoresist in order to increase the solubility rate of the resist exposed thereto, particularly including the exposed sidewalls of the resist layer opening. Assuming that light is used to flood the resist layer from above, then the amount of light applied when flooding from above is preferably greater than the amount of light applied to flood the lowermost region of the resist layer before the first development step. The upper surface of the resist layer is then cured by exposure to a high dosage, low voltage (i.e., 30 KV or less) electron beam, deep ultraviolet light, or the like, to render such exposed regions relatively insoluble. Such electron beam or deep ultraviolet light is applied at a dosage sufficient to cause the uppermost region of the resist layer to become cross-linked. A second development step is then performed to dissolve exposed portions of the sidewalls of the patterned opening, except for the cured uppermost region of the positive resist layer, to form an undercut resist sidewall profile.
When using the aforementioned method as part of a metal lift-off process to form a patterned metal layer upon the substrate, a layer of metal is deposited over the patterned resist layer, and over exposed portions of the substrate. The resist layer is then stripped off the upper surface of the substrate, leaving the patterned metal layer on the surface of the substrate.